1. Field of the Invention
The invention pertains to the field of variable cam timing system. More particularly, the invention pertains to a variable cam timing system with variable chamber volume.
2. Description of Related Art
Cam torque actuated (CTA) phasers use torque reversals in the camshaft, caused by the forces of opening and closing engine valves to move the vane. Control valves are present to allow fluid flow from chamber to chamber causing the vane to move, or to stop the flow of oil, locking the vane in position. The CTA phaser has oil input to make up for losses due to leakage, but does not use engine oil pressure to move the phaser. CTA phasers have shown that they provide fast response and low oil usage, reducing fuel consumption and emissions. However, in some engines, i.e. 4-cylinder engines, the torsional energy from the camshaft is not sufficient to actuate the phaser over the entire speed range of the engine, especially when the rpm is high and optimization of the performance of the phaser in view of engine operating conditions (e.g. the amount of available cam torque) is necessary.
FIGS. 1a through 1c show a conventional cam torque actuated phaser (CTA). Torque reversals in the camshaft caused by the forces of opening and closing engine valves move the vane 106. The advance and retard chambers are arranged to resist positive and negative torque pulses in the camshaft and are alternatively pressurized by the cam torque. The control valve 104 in a CTA system allows the vane 106 in the phaser to move, by permitting fluid flow from the advance chamber 108 to the retard chamber 110 or vice versa, depending on the desired direction of movement, as shown in FIGS. 1a and 1b. Positive cam torsionals are used to retard the phaser, as shown in FIG. 1a. Negative cam torsionals are used to advance the phaser, as shown in FIG. 1b. A null or central position, as shown in FIG. 1c, stops the flow of fluid, locking the vane in position.
More specifically, in moving towards the retard position of the phaser, as shown in FIG. 1a, the spool valve 104 is internally mounted within the rotor and includes a sleeve 117 for receiving a spool 109 with lands 109a, 109b, 109c and a biasing spring 105. A variable force solenoid (VFS) 103, which is controlled by an ECU 102, moves the spool 109 within the sleeve 117. In moving towards the retard position, as shown in FIG. 1a, the force of the VFS 103 was reduced and the spool 109 was moved to the left by spring 105, until the force of the spring 105 balanced the force of the VFS 103. In the position shown, spool land 109b blocks line 113, and lines 112 and 116 are open. Camshaft torque pressurizes the advance chamber 108, causing fluid in the advance chamber 108 to move into the retard chamber 110. Fluid exiting the advance chamber 108 moves through line 112 and the fluid moves and into the spool valve 104 between spool lands 109a and 109b. From the spool valve 104, fluid move back into line 116 where it feeds into line 113 supplying fluid to the retard chamber 110. As stated earlier positive cam torsionals are used to aid in moving the vane 106.
Makeup oil is supplied to the phaser from supply S to make up for leakage and enters line 118 and moves through inlet check valve 119 to the spool valve 104. From the spool valve fluid enters line 116 through either of the check valves 114, 115, depending on which is open to either the advance chamber 108 or the retard chamber 110.
To move towards the advance position of the phaser, as shown in FIG. 1b, the force of the VFS 103 was increased and the spool was moved to the right by the VFS 103, until the force of the spring balances the force of the VFS 103. In the position shown, spool land 109a blocks the exit of fluid from line 112, and lines 113 and 116 are open. Camshaft torque pressurizes the retard chamber 110, causing fluid in the retard chamber 110 to move into the advance chamber 108. Fluid exiting the retard chamber 110 moves through line 113 and into the spool valve 104 between lands 109a and 109b. From the spool valve 104, the fluid enters line 116 and travels through open check valve 114 into line 112 and the advance chamber 108. As stated earlier only negative cam torsionals are used to move the vane 106.
Makeup oil is supplied to the phaser from supply to make up for leakage and enters line 118 and moves through inlet check valve 119 to the spool valve 104. From the spool valve fluid enters line 116 through either of the check valves 114, 115, depending on which is open to either the advance chamber 108 or the retard chamber 110.
FIG. 1c shows the phaser in null or a central position where the spool lands 109a, 109b block lines 112 and 113, respectively and vane 106 is locked into position. A small amount of fluid is provided to the phaser to make up for losses due to leakage.
U.S. Pat. No. 4,809,650 discloses a variable volume chamber in which the surface area of the chambers does not change, but the volume of fluid present does. Hydraulic fluid is fed into a variable volume chamber defined between an outer piston and an inner piston which is reciprocatively disposed therein, via a supply passage which includes a one-way valve. A valve chamber is present between the two chambers and contains a spool valve. When low compression engine operation is required, the pressure is supplied to the valve chamber, moving the spool valve so that the supply passage is closed. Transfer passages and a drain passage are open. The drain passage leads directly to the cylinder bore so that hydraulic fluid in the variable volume chamber is vented in an unrestricted manner. U.S. Pat. No. 4,934,347 is similar and discloses a damping device that is a small diameter piston-like valve element in a coaxial bore in the large land of the spool.
U.S. Pat. No. 5,823,152 discloses a rotor and a housing that define chambers whose volumes are variable in accordance with rotational position of the rotor with respect to the housing, but the surface area remains the same. The vanes have drain holes. A stopper piston serves as a locking member and is housed in the vane. A switching valve directs fluid to the chambers. Other examples of a rotor and a housing that define chambers whose volumes are variable in accordance with rotational position of the rotor with respect to the housing, but the surface area remains the same, include U.S. Pat. No. 6,155,221 and U.S. Pat. No. 6,199,524.
U.S. Pat. No. 6,389,809 discloses a volume control valve for controlling the volume of a variable displacement type hydraulic rotary machine. The volume control valve includes a housing with a bore for receiving a spool that selectively blocks communication of a pressure oil feed/discharge port with a high pressure port and a tank port. A first pressure receiving portion is formed in the spool to receive a load pressure as a pilot pressure is introduced for displacing the spool axially within the bore. The volume control valve is selectively controlled using an external command pressure. When the external command pressure is down to the tank pressure, the spool maintains the position regardless of the pilot pressure introduced from the pilot port and the volume control valve is fixed at a large volume. When the external command pressure is increased to displace the spool in the opposite direction, the spool slides in the direction in accordance with the pilot pressure of the hydraulic rotary machine. In this state, the spool receives an external command pressure in the opposite direction and the volume control can make a selective volume control by utilizing the difference between the external command pressure and the pilot pressure. Again, the surface area of the variable displacement type hydraulic rotary machine does not change.
Therefore, there is need for a phaser that optimizes the phaser with respect to engine conditions and varies the surface area of the chambers to ensure that the chambers have a suitable surface area as required by the engine for optimum performance.